Does detecting water vapor on rocky planets indicate the presence of oceans?

Criteria for habitability are often discussed in terms of the habitable zone, but in addition, a sufficient amount of water is required at the surface to develop water oceans. The majority of the water inventory could remain in the mantle during magma ocean solidification, and mantle degassing during the subsequent period is limited for planets operating under stagnant lid convection. In this study, we calculate the rate of mantle degassing during magma ocean and in the subsequent solid-state convection stage to discuss the likelihood of ocean formation. The results show that the most important criterion for ocean formation is the H/C ratio of the initial mantle composition, and a threshold ratio increases with planetary size and net stellar radiation. Larger planets are less likely to develop water oceans because a greater amount of greenhouse gases are emitted to the atmosphere during magma ocean, and hydrogen escape would hinder ocean formation for planets receiving stronger radiation. Volatile concentrations in the bulk silicate Earth are close to the threshold amount for ocean formation, so even with similar volatile compositions to Earth, all surface water could exist as vapor, and water oceans may be absent when exoplanets are larger or/and receive stronger stellar radiation compared to Earth. The existence of water oceans is therefore not guaranteed even for exoplanets located in the habitable zone. Despite its difficulty in detection, smaller exoplanets located further away from the central star may be a better candidate to search for biosignatures. Our results also provide a plausible explanation for the divergent evolutionary paths of Earth and Venus.